The present invention relates in general to a converter control apparatus, and more particularly to a control apparatus for a converter circuit constituted by unit converters connected in cascade in a multiplicity of stages.
In general, an input current to a phase-controlled converter has a fundamental wave whose magnitude remains constant independent of the output voltage with only the power-factor angle varying. As the result, when the output voltage of the converter is lowered, a reactive power is increased, involving degradation in the power factor.
As a method of decreasing the reactive power, there is known, by way of example, a so-called asymmetrical control system in which individual converters are connected in cascade and operated in the vicinity of a maximum voltage of positive or negative polarity. For example, reference may be made to "APPLICATION OF NEW DRIVE SYSTEMS FOR PLATE MILL DRIVES" (IEEE-lAS84: 38, p.p. 43-48, 1984) and "MICROCOMPUTER-BASED HIGH CAPACITY CYCLOCONVERTER DRIVE FOR MAIN ROLLING MILL" (IPEC-Tokyo '83, p.p. 744-755). In these literatures, it is described that one stage of the cascaded converters is fixed at a maximum output voltage, while the output voltage of the other converter stage is regulated for controlling the output voltage of the cascaded converter circuit in a low voltage region (i.e. region in which the power factor is low).
As will be seen, the asymmetrical control system for the cascaded converter circuit known heretofore is designed to improve the power factor of the power supply source by setting the operating point of the unit converter, i.e. the base level of the sinusoidal output component of the unit converter, in the vicinity of the maximum voltage of positive or negative polarity. Consequently, a ripple component in the output voltage of the cascaded converter circuit varies in dependence on the firing phase or angle, giving rise to a phenomenon that the voltage ripple component is increased, which in turn results in generation of significant torque ripple and electromagnetic noise in case the load is an electric motor.